Cathode



Sept. 8, 1964 BRODIE ETAL CATHODE Filed Aug. 10, 1962 IIIIIIIJII r/////////// ///////lr////// $8... R N Y 085 E T a N MR R E m Fm W R T m A e n 8 B WITNESSES flzw *4 2 States Unite l This invention relates to matrix type cathodes and, more particularly, to a method of manufacturing thermionic cathodes of the matrix type.

One method by which the matrix type cathode is manufactured is by mixing an electron active material, for example the carbonates of barium, strontium and calcium in powder form with finely divided nickel powder. (In the ensuing discussion, the expression electron active material as herein used is defined to mean a material which is the source of the requisite electron emissive material, for example barium. Also, nickel powder as herein used is understood to include nickel materials well known in the electron tube art which may include impurities such as carbon, copper, iron, sulfur, titanium, aluminum, manganese, silicon and magnesium in amounts up to about 1 percent. Cathode grade nickel, for example, which has been found to provide excellent results in this type cathode, contains impurities in an amount of about 0.5 percent. Nickel powder then, is meant to include powders comprised substantially of nickel.) The resultant mixture is then pressed into a retaining body at high pressures (approximately 80 to 90 tons per square inch). A backing layer of nickel powder containing a reducing agent may be simultaneously formed in the same pressing operation as set forth above. The high pressures used in forming the cathode cause the particles of nickel powder to flow together or cold weld and hence form a coherent matrix with the carbonates. This matrix shows very little porosity. During subsequent processing, the cathode is heated to convert the carbonates into oxides with the evolution of carbon dioxide gas. Due to the lack of porosity, it has been found that prolonged heating at high temperatures is necessary in order to remove the last traces of gas. Also, it is known that for the ordinary oxide coated cathode to emit well, it is necessary to have a very porous coating. For example, the density of the oxide coating on customary coated type cathodes is usually about one-quarter that of the bulk oxides.

It is, therefore, an object of this invention to provide an improved method for the manufacture of thermionic cathodes.

Another object is to provide an improved method for the manufacture of thermionic cathodes of the matrix A further object is to provide an improved manufacturing method for matrix type cathodes of a porous nature.

A still further object is to provide an improved means for manufacturing a matrix type cathode of the desirable porosity but to retain the simple manufacturing technique of the ordinary matrix cathode.

Stated briefly, the present invention provides that prior to the mixing of an electron active material with finely divided nickel powder, the electron active material is coated with an evaporable additive which can be removed from the cathode at a lower temperature than is required to activate the electron active material. If the electron active material is coated or enclosed in the additive, then the resultant porosity, when the additive is removed, will be only in the region of the electron active material and will not affect the nickel supporting matrix.

Further objects and advantages of the invention will become apparent as the following description proceeds and features of novelty which characterize the invention 3,148,56 Patented Sept. 8, 1964 ice will be pointed out in particularity in the claims annexed to and forming a part of this specification.

For a better understanding of the invention, reference may be had to the accompanying drawings in which:

FIG. 1 shows a sectional view of a matrix type cathode in accordance with the present invention; and,

FIG. 2 is a greatly enlarged view in section illustrating the mixture utilized to form the cathode matrix of FIG. 1.

With reference now to FIG. 1, there is shown a matrix type dispenser cathode such as is produced by the method of the present invention. The cathode comprises a sleeve 10 which is of a suitable material such as nickel which includes at its upper portion a reentrant portion 12. Disposed within the reentrant portion 12 is a layer 14 which is comprised of powdered nickel mixed with a small quantity of reducing agent which during the life of the cathode diffuses into a matrix 16 positioned on top of the layer 14. The reducing agent reacts with an electron active material compound within the matrix 16 to produce a free element necessary for the operation of the cathode. A heater element 18 is disposed within the bottom of the sleeve 10 below the reentrant portion 12.

The present invention, which produces the matrix 16 of FIG. 1, may best be explained with reference to FIG. 2. In FIG. 2, there is shown a mixture of finely divided nickel powder 20 and powdered electron active material 22 which has been supplied with an evaporable resinous coating 24. The electron active material utilized in this invention is one or more of the alkaline earth metal emission compounds and is, preferably, one or more of the carbonates of barium, strontium, and calcium. The resin utilized as the coating 24 in the present invention is an acrylic resin and preferably a methacrylate polymer.

One method by which the material 22 may be coated is by dissolving the resin in a suitable solvent such as toluene and adding the material 22 to this solution. The solvent is then removed, for example by boiling, while the mixture is being continuously stirred. After the solvent has been removed, the residue is thoroughly dried, for example by heating in a vacuum oven, and is then powdered by suitable means such as grinding. This results in a powdered electron active material which is coated with the resin.

The coated electron active material is now mixed with finely divided nickel powder and by the application of pressure in the range of from to tons per square inch is formed into a coherent matrix. In the present invention, the matrix 16 may be formed independently or it may be formed within the sleeve 16. After the matrix is formed, it is heated to remove the resinous coating 24 with the result that the matrix has a porosity only in the vicinity of the electron active material. This porosity facilitates the removal, from the matrix, of any unwanted material. For example, if carbonates are used, they are heated to produce oxides with the evolution of carbon dioxide gas. The porosity of the present matrix readily allows the removal of the carbon dioxide.

In one specific cathode made in accordance with the present invention, Lucite 44 (n-buty1-methyl-methacrylate) was utilized as the resinous coating. The Lucite was dissolved in toluene and one or more of the carbonates of barium, strontium and calcium were added to the above solution. The toluene was then boiled oif while the mixture was being continuously stirred and the residual material was then thoroughly dried in a vacuum oven at C. After drying, the solids were powdered with a pestle and mortar and passed through a 200 mesh sieve to select powder of optimum size. In the above, the

Lucite and carbonates were combined in a ratio of approximately 0.825 gram of Lucite to each gram of carbonate. This produces a final density within the matrix which is roughly the same as that found in the customary oxide coated cathode.

The mixture for the matrix 16 (FIG. 1) was then made with a ratio of 70 percent by weight nickel and 30 percent by weight of the Lucite-carbonates. The cathode utilized a backing layer 14 (FIG. 1) of nickel powder having about 0.2 percent silicon by weight as the activator. In the device as above described, after the Lucite 44 was removed by heating to a temperature of about 400 C., the matrix 16 was again heated, this time to a higher temperature to reduce the carbonates to oxides, with the resulting evolution of carbon dioxide gas. This reduction occurs generally in the approximate range of temperatures from about 1175 to about 1250 C. In this device, the carbon dioxide gas was more easily released than in prior matrix cathodes. It was found that all traces of carbon dioxide were removed after application of current of 1 ampere for 1 minute to the cathode heater. This compares with times of 3 to 5 minutes for standard matrix cathodes after which time gases are still evolving slowly from the cathode. In addition to the more rapid evolution of the carbon dioxide gas, the porous cathode of the present invention was found to be immediately active whereas prior art matrix cathodes often require a long aging time before reaching their full emission, normally in the range of from 20 to 30 hours.

The cathode as above described was found to have, in comparison to prior matrix cathodes, improved emission stability and long life.

While there have been shown and described what are at present considered to be the preferred embodiments of the invention, modifications thereto will readily occur to those skilled in the art. It is not desired, therefore, that the invention be limited to the specific arrangements shown and described and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

We claim as our invention:

1. A method of producing a thermionic cathode for the emission of electrons comprising the steps of providing a quantity of electron active material powder, coating said powder with an evaporable resin material, admixing said coated powder with a quantity of powder comprised substantially of nickel to provide a mixture thereof, forming a coherent matrix of said mixture by the application of pressure thereto, and heating said matrix to remove resinous coating to provide that said matrix is porous only in the region of said electron active material.

2. A method of producing a thermionic cathode for the emission of electrons comprising the steps of providing a quantity of alkaline earth metal emission compounds in powdered form, coating said powder with an evaporable resin material, admixing said coated powder with a quantity of powder comprised substantially of nickel to provide a mixture thereof, forming a coherent matrix of said mixture, and heating said matrix to remove said resinous coating to provide that said matrix is porous in the region of said alkaline earth metal compounds.

3. A method of producing a thermionic cathode for the emission of electrons comprising the steps of providing a quantity of alakline earth metal emission compounds in powdered form, coating said powder with an acrylic resin, admixing said coated powder with a quantity of powder comprised substantially of nickel to provide a mixture thereof, forming a coherent matrix of said mixture, and heating said matrix to remove said acrylic resin to provide that said matrix is porous in the region of said alkaline earth compounds.

4. A method of producing a thermionic cathode for the emission of electrons comprising the steps of providing a quantity of alakline earth metal emission com pounds in powdered form, coating said powder with a methacrylate polymer, admixing said coated powder with a quantity of powder comprised substantially of nickel to provide a mixture thereof, forming a coherent matrix of said mixture, and heating said matrix to remove said methacrylate polymer to provide that said matrix is porous in the region of said alkaline earth compounds.

5. A method of producing a porous matrix thermionic cathode comprising the steps of providing in powdered form a quantity of at least one carbonate selected from the group consisting of barium, strontium and calcium, carbonate coating said powder with an evaporable material, admixing said coated powder with powder comprised substantially of nickel to form a mixture thereof, forming a coherent matrix of said mixture by the application of pressure thereto, and heating said matrix to remove said evaporable coating to provide that said matrix is porous only in the region of said carbonate.

6. A method of producing a porous matrix thermionic cathode comprising the steps of providing in powdered form a quantity of at least one carbonate selected from the group consisting of barium, strontium and calcium, carbonates coating said powder with an acrylic resin, admixing said coated powder with powder comprised substantially of nickel to form a mixture thereof, forming a coherent matrix of said mixture by the application of pressure thereto, and heating said matrix to remove said acrylic resin to provide that said matrix is porous only in the region of said carbonate.

7. A method of producing a porous matrix thermionic cathode comprising the steps of providing in powdered form a quantity of at least one carbonate selected from the group consisting of barium, strontium and calcium, carbonates coating said powder with a methacrylate polymer, admixing said coated powder with powder comprised substantially of nickel to form a mixture thereof, forming a coherent matrix of said mixture by the application of pressure thereto, and heating said matrix to remove said methacrylate polymer to provide that said matrix is porous only in the region of said carbonate.

8. A method of producing a porous matrix type thermionic cathode comprising the steps of providing in powdered form a quantity of at least one carbonate selected from the group consisting of barium, strontium and calcium, carbonates coating said powder with n-butylmethyl-methacrylate, admixing said coated powder with a finely divided powder comprised substantially of nickel to form a mixture thereof, forming a coherent matrix of said mixture by the application of sufiicient pressure thereto to provide that said nickel powder is cold welded together, and heating said matrix to a temperature and for a period of time sufficient to evaporate said n-butylmethyl-methacrylate from said matrix.

9. A method of producing a porous matrix type themionic cathode comprising the steps of providing in powdered form a quantity of at least one carbonate selected from the group consisting of barium, strontium and calcium, carbonates coating said powder with a methacrylate polymer; said coating comprising the steps of dismethacrylate polymer in a solvent, adding said powder carbonate to said solution, evaporating the solvent while continuously stirring the mixture of carbonate and solution, and powdering the resulting methacrylate polymer coated carbonate; admixing said coated carbonate with a finely divided powder comprised substantially a coherent matrix of said mixture by the application of sufiicient pressure to said mixture to provide that said nickel powder is cold welded together, and heating said matrix to a temperature and for a period of time sufficient to evaporate said methacrylate polymer from said matrix.

10. A method of producing a porous matrix thermionic cathode comprising the steps of providing in powdered form a quantity of at least one carbonate selected from the group consisting of barium, strontium and calcium, carbonates coating said powder carbonate with a methacrylate polymer, admixing said coated powder carof nickel to form a matrix thereof, forming bonate with a finely divided powder comprised substantially of nickel in the proportion by weight of about 30% coated powder carbonate to 70% nickel powder to form a mixture thereof, forming a coherent matrix of said mixture by the application of pressure thereto, and heating said matrix to remove said methacrylate polymer to provide that said matrix is porous only in the region of said carbonate.

11. A method of producing a porous matrix therrn ionic cathode comprising the steps of providing in powdered form a quantity of at least one carbonate selected from the group consisting of barium, strontium and calcium, carbonates coating said powder carbonate with a methacrylate polymer, said carbonate and said polymer being in a weight ratio respectively of about 1 to 0.85, admixing said coated powder carbonate with finely di- References Cited in the file of this patent UNITED STATES PATENTS 1,988,861 Thorausch Jan. 22, 1935 2,928,733 Wagner Mar. 15, 1960 FOREIGN PATENTS 818,051 Great Britain Aug. 12, 1959 

1. A METHOD OF PRODUCING A THERMIONIC CATHODE FOR THE EMISSION OF ELECTRONS COMPRISING THE STEPS OF PROVIDING A QUANTITY OF ELECTRON ACTIVE MATERIAL POWDER, COATING SAID POWDER WITH AN EVAPORABLE RESIN MATERIAL, ADMIXING SAID COATED POWDER WITH A QUANTITY OF POWDER COMPRISED SUBSTANTIALLY OF NICKEL TO PROVIDE A MIXTURE THEREOF, FORM- 